2.5, 3.5, 9, 40, ?

Find the next number in the above-given series?

343 = 73,  1331 = 113 , 2197 = 133, 121 = 112

343, 1331, 2197 are perfect cubes, but not 121.

The digits on either side of the letter in each of the groups 1P6, 2T0, and 1R8 indicate the place value of the letter in the alphabet.

This pattern is not followed by 2Y4.

K+2 M+1 N-2 L,  P+2 R+1 S-2 Q,   V+1 W+2 Y+1 Z,   J+2 L+1 M-2 K  and  W+2 Y+1 Z-2 X.

Except VWYZ, all the other groups follow similar pattern.

The series follows the following pattern

5 X 5 – 5 = 20

20 X 4 – 4 = 76

76 X 3 – 3 = 225

225 X 2 – 2 = 448

Hence, the term 225 denoted by (d) completes the series

With close observation, you will note that each number in the list is in the middle of two prime numbers. Thus:

4 is in the middle of 3 and 5, 6 is in the middle of 5 and 7, 12 is in the middle of 11 and 13, 18 is in the middle of 17 and 19, 30 is in the middle of 29 and 31. 42 is in the middle of 41 and 43, 60 is in the middle of 59 and 61.

Therefore, the next number would be the one that is in the middle of the next two prime numbers, which is 72 (which is in the middle of 71 and 73).

There should be 90 instead of 80
so we can have a series
5, 5x3, 15x2, 30x3, 90x2, 180x3, 540x2 and 1080x3 and so on...

Here the series follow the rule that

12+22 = 5

32+42 = 25

52+62 = 61

72+82 = 113

SO NEXT IS, 92+102 = 181

3, 7, 6, 5, 9, 3, 12, 1, 15, ... has two different series

1) 3, 6, 9, 12, 15.....

2) 7, 5, 3, 1, .....

-1 is the next number.